1. Field of the Invention
The present invention relates to composite materials and a method of controlling damage to composite materials, and a damage sensor for detecting damage to composite materials.
2. Description of Prior Art
Composite materials are used for next-generation aircraft, satellites, space stations, skyscrapers, public infrastructures, high-speed vehicles, and so on.
Design tolerance for composite materials is set about one-fourth of the strength which the materials have by nature because the materials are prone to damage due to impact loads.
There are new types of composite materials to raise design tolerance for safety improvement. A material that suppresses damage or a sensor that detects an impact load and damage, is embedded in the composite materials.
Japanese Unexamined-Patent Publication No. 1994(6)-212018 discloses a polymer-based advanced composite material. The material has at least one type of shape-memory alloys arranged over or inside the raw material of the composite material. Each shape-memory alloy has been deformed at a temperature equal to or lower than a specific temperature at which reverse transformation completes.
Japanese Unexamined-Patent Publication No. 1995(7)-48637 discloses a metal-based composite material. The material has at least one type of shape-memory alloys mixed or arranged inside the raw material of the composite material. Each shape-memory alloy exhibits thermoelastic transformation.
Japanese Unexamined-Patent Publication No. 1996(8)-15208 discloses a damage detection system for composite materials. A fine wire of NiTi-shape-memory alloy is embedded in laminated composite materials. A current flows through the fine wire to detect change in electrical resistance of the fine wire, which will occur when composite materials crack, for example.
The shape-memory alloys disclosed by the Publication Nos. 1994(6)-212018 and 1995(7)-48637 may become foreign substances to composite materials after embedded to cause false defects, thus weakening the strength of the materials.
These publications disclose usage of NiTi-shape-memory alloys as an actuator to suppress development of cracks on composite materials. Also taught is a strain gage and a piezoelectric transducer that can be used as a damage sensor. However, there is no disclosure of how to use these sensors.
The damage detection system disclosed by the Publication No. 1996(8)-15208 uses different materials for damage detection and suppression. A shrinkage uniformly occurring to a fine wire of shape-memory alloy suppresses transverse cracks.
This system, however, hardly detects change in the characteristics of the fine wire when the change becomes small as the wire lengthens, and also hardly locates the position of damage to the composite material, which causes the change.
Moreover, this system cannot protect composite materials from delamination which tends to occur due to impact loads when the materials have a low design tolerance.